This invention relates generally to gas turbine engines, and specifically to turbine and compressor airfoils. In particular, the invention concerns a maintenance method for turbine airfoils with coated tip structures, including abrasive airfoil tip coatings configured to form rotary air seals.
Gas turbine engines (or combustion turbines) are built around a power core made up of a compressor, combustor and turbine, arranged in flow series with an upstream inlet and downstream exhaust. The compressor compresses air from the inlet, which is mixed which fuel in the combustor and ignited to generate hot combustion gas. The turbine extracts energy from the expanding combustion gas, and drives the compressor via a common shaft. Energy is delivered in the form of rotational energy in the shaft, reactive thrust from the exhaust, or both.
Gas turbine engines provide efficient, reliable power for a wide range of applications, including aviation, industrial power generation, and commercial heating and cooling. Small-scale engines such as auxiliary power units typically utilize a one-spool design, with co-rotating compressor and turbine sections. Larger-scale engines including jet engines and industrial gas turbines (IGTs) are generally arranged into a number of coaxially nested spools, which operate at different pressures and temperatures, and rotate at different speeds.
The individual compressor and turbine sections in each spool are subdivided into a number of stages, which are formed of alternating rows of rotor blade and stator vane airfoils. The airfoils are shaped to turn, accelerate and compress the working fluid flow, and to generate lift for conversion to rotational energy in the turbine.
Ground-based industrial gas turbines can be quite large, utilizing complex spooling systems for increased efficiency. Power is delivered via an output shaft connected to a mechanical load, such as an electrical generator, blower or pumping system. Industrial turbines can also be configured for combined-cycle operation, in which additional energy is extracted from the exhaust stream, for example in a low pressure steam turbine.
Aviation applications include turbojet, turbofan, turboprop and turboshaft engines. Turbojet engines are an older design, in which thrust is generated primarily from the exhaust. Modern fixed-wing aircraft generally employ turbofan and turboprop engines, in which the low pressure spool is coupled to a propulsion fan or propeller. Turboshaft engines are used on rotary-wing aircraft, including helicopters.
Turbofan engines are commonly divided into high and low bypass designs. High bypass turbofans generate thrust primarily from the fan, which drives airflow through a bypass duct oriented around the engine core. Low-bypass turbofans generate proportionally more thrust from the exhaust flow. This design provides greater specific thrust but incurs costs in noise and fuel efficiency, and is primarily used on supersonic fighters and other high-performance aircraft. Unducted (open rotor) turbofans and ducted turboprop engines are also known, including counter-rotating and aft-mounted configurations.
Gas turbine engine performance requires maintaining precise clearance between the various rotating and stationary components, particularly for rotor blade and stator vane airfoils. In the airfoil tip region, abrasive/abradable coatings are sometimes used to provide a rotating seal, increasing efficiency by reducing blow-by and other losses. Abrasive seals can be service-limiting components, however, typically requiring regular inspection and service.